1. Field of the invention
The present invention relates to an error correcting apparatus for an image processing system, and more particularly to an apparatus for correcting a coded image signal which is transmitted from an image coding system and in which an error signal is contained.
2. Description of the Prior Art
Generally, an image processing system is constituted by two main circuit parts, one of which is an image coding part for coding an image signal to generate a coded image signal, and the other of which is an image decoding part for decoding the coded image signal from the image coding part. When an image signal is transformed into a coded image signal by means of the image coding part, an error arises because of variation of power, circuit defect in the coding part, noises on transmitting lines and the like. Accordingly, the image coding part further has an error code generator in order to detect occurrence of error and generate an error correction code, and the image decoding part also has an error code decoder for decoding the error correction code in order to reproduce original image signal without error.
Since information of image signals are large in quantity, it is necessary to compress and transmit information of the image signals by means of an image coding system. Most of image coding systems which have been recently developed are a prerequisite to use a variable length coding method so as to effectively compress image information. By using the variable length coding method, image information can be effectively compressed, but when an error is generated in the compressed image information due to occurrence of noises on transmitting lines and the like, this error has a bad influence upon following image information, so that the compressed image information can not be reproduced near to original image information.
For this reason, most of image coding systems each have an error correction circuit, such as an error code generator, so as to reduce the frequency number in occurrence of error. However, since a burst error, which can not be corrected, can be risen even in such an error correction circuit, it is necessary to divide and transmit all the image information into the amount of information corresponding to every a predetermined period so as to reduce the range of information affected by an error. Therefore, even through an error arises, the error has an influence upon only a predetermined range of the divided image information, so that the range of image information affected by the error can be limited within the range corresponding to the divided image information of error occurrence. However, the smaller the range affected by an error, the lower the compression efficiency.
When the image information are compressed, good quality of image can be obtained only if all portions in the image have uniform image quality. in this case, the amount of compressed information is determined in accordance with each of the portions in the image.
In case that a variable length coding method is provided for coding image information, a method that the amount of coded image information make to be set equally every period by compulsion is utilized in such an image coding system. Generally, the amount of coded image information is small in portion where variation of image is relatively small, but large in portion where variation of image is relatively large. Accordingly, if the amount of coded image information is set equally every period by compulsion, quality of the image is seriously lowered in the portion where the variation is relatively large. As mentioned above, even through only a portion of the image is relatively distorted in image quality, as compared to other portions, it appears to human as if overall portions of the image are badly distorted because of characteristic of eyesight.
Therefore, if a scrambling method well-known in the art is provided for coding an image, which can rearrange image information to make all the portions in its image uniformly, image quality becomes uniform even if the amount of the coded image information is equally set every period, so that distortion of the image quality can be prevented.